Paint and coatings
Paint calculator
How many gallons the room actually takes — with the spread rate adjusted for what you are painting over, and the first coat counted separately from the ones after it.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-16 · Nothing you type leaves your browser
What are you coating?
Room 1
12' 6", 12.5 and 12-6 all work
Eight feet unless the house says otherwise.
Subtracted from the wall, and painted separately if you say so.
The condition the spread rate on the tin assumes. Two coats and you are done.
Covers in two coats on a sound surface and scrubs without burnishing. The sensible default for most rooms.
Two is the realistic default. One only works over a similar colour.
Optional here.
Used only to pick a regional labour rate. It stays in your browser.
Wall paint
3 gallons
461 ft² of wall after 51 ft² of openings · 2 coats · first coat covers 400 ft²/gal, later coats 400
Both coats cover about 400 ft² a gallon here
Painting over a similar colour on a sealed wall is the best case, and the published spread rate holds. Almost every other situation on the list above costs you more paint in the first coat.
Shopping list
- Wall paint, mid range
461 ft² of wall, 2 coats3 gallons - Ceiling paint
252 ft² — flat white, different product from the walls2 gallons - Trim and door paint
Bought in quarts — a gallon of trim paint is rarely used before it skins over2 quarts - Sundries
Rollers, sleeves, a brush worth using, tape, drop cloths, filler, sandpaper and a tray. It is a fixed cost per job that scales with rooms, not with area.≈ $60
What it costs
National average. Paint, primer, sundries and professional labour.
Doing it yourself removes $1,504 of that — painting is the trade with the widest gap between material and labour on this site, which is exactly why it is the job most people take on themselves.
On this page — 9 sections
- 01Reading the result
- 02Why the number on the tin is the best case
- 03When primer is worth it and when it is not
- 04Openings, trim and the things sold separately
- 05What you are buying when you pay more
- 06Garage floor epoxy, where the coating is the cheap part
- 07Five ways this goes wrong
- 08How the calculator works it out
- 09The words on the tin
Reading the result
The short version
- 400 ft² a gallon is the best case, not the average. It assumes a smooth, sealed, previously painted wall.
- The shortfall is almost entirely in the first coat, so the calculator counts the coats separately instead of averaging them.
- Primer is not an upsell where the surface needs it — it costs less than the extra finish coat it replaces.
- Painting has the widest material-to-labour gap of any trade here. Doing it yourself saves most of the bill.
The headline is gallons of wall paint. Under it, the working shows the wall area after openings, the number of coats, and — the part that matters — the spread rate for the first coat against the ones after it.
That split is the whole point of this page. Every other paint calculator divides your area by a single number. Real surfaces do not behave that way, and the moment you notice is usually halfway through the first coat on a Saturday.
gallons = area ÷ first-coat rate + area × (coats − 1) ÷ later-coat rate
- area
- walls after doors and windows come out
- first-coat rate
- 400 ft² adjusted for what you are painting over
- later-coat rate
- close to the published figure, because the surface is now sealed
Worked through — A room with 340 ft² of wall, two coats, on new drywall: 340 ÷ 200 = 1.7 gal for the first coat, plus 340 ÷ 380 = 0.9 for the second. That is 2.6 gallons — where a single 400 ft²/gal assumption would have told you 1.7.
Why the number on the tin is the best case
Manufacturers are not being dishonest when they print 400 square feet per gallon. They are quoting a measured figure under defined conditions: a smooth, non-porous, previously painted surface, applied at the recommended film thickness.
Those conditions describe repainting a wall that is already in good shape. They do not describe new drywall, texture, bare wood or masonry — and each of those falls short in a different way and by a different amount.
| Painting over | First coat | Later coats | Why |
|---|---|---|---|
| Previously painted, similar colour | 400 ft² | 400 ft² | The condition the spread rate on the tin assumes. Two coats and you are done. |
| Previously painted, big colour change | 380 ft² | 400 ft² | Going light over dark, or covering a strong colour. Tinted primer beats a third coat of finish paint, and costs less. |
| New drywall | 200 ft² | 380 ft² | Bare paper and joint compound absorb at completely different rates. Without primer the joints show through as a dull pattern the paint cannot fix. |
| Textured walls or ceiling | 300 ft² | 320 ft² | Knockdown and orange peel add real surface area you cannot see. Expect a quarter more paint than the flat equivalent. |
| Bare wood | 220 ft² | 360 ft² | Wood keeps absorbing until it is sealed, and knots bleed through anything that is not a stain-blocking primer. |
| Masonry, brick or stucco | 180 ft² | 300 ft² | The most absorbent surface here by a distance. Porous, rough and thirsty — budget roughly double the smooth-wall figure. |
Sherwin-Williams, Interior and exterior coatings product data sheets · checked 2026-09-16
The full coverage table, free to cite →When primer is worth it and when it is not
Primer gets sold as a default and skipped as a shortcut, and both are wrong. It does three specific jobs, and if none of them apply you do not need it.
It evens out absorption. New drywall is bare paper next to joint compound, and the two drink paint at completely different rates. Finish paint over that dries patchy — dull over the joints, sheen over the paper — and no number of extra coats fixes it, because each one does the same thing again.
It blocks. Water stains, smoke, tannin from knots in bare wood. These bleed through finish paint indefinitely; a stain-blocking primer stops them once.
It grips. On a glossy surface, finish paint has nothing to hold onto. A bonding primer gives it something.
Where none of that applies — repainting an eggshell wall a similar colour — primer buys you nothing that a coat of the finish paint would not.
| Prime, then two coats | Three coats of finish | |
|---|---|---|
| Cost per gallon | Primer is cheaper than finish paint | Full price every time |
| Evens absorption | Yes — that is its job | No. Each coat absorbs unevenly again |
| Blocks stains | Yes, with the right primer | No. They keep coming through |
| Big colour change | Tinted primer gets you most of the way | Works, but takes the third coat and often a fourth |
| Time | Primer dries fast and sands easily | Longer between coats |
Openings, trim and the things sold separately
A door is about 21 square feet of wall you do not have to paint, and a window around 15. In a small room with two windows and a door that is fifty square feet — a meaningful share of a gallon.
It does not save proportional time, though, and this is where DIY estimates and professional quotes diverge. Rolling a flat wall is fast. Cutting in around a window frame is slow, and you have added an edge by removing the area. A room full of openings costs a painter more per square foot, not less.
Trim and doors are a different product as well as a different surface: semi-gloss rather than eggshell, in quarts rather than gallons. A gallon of trim paint almost always skins over in the tin long before it is used up, which makes quarts cheaper in practice despite costing more per unit.
21 ft²
Per door
Subtracted from the wall
15 ft²
Per window
Average of a mixed bag
40 ft²
Painting a door
Both faces and the edges
0.6 ft²
Per foot of trim
Baseboard and casing together
Bought once, forgotten every time
- A brush worth cleaning rather than three that are not.
- Roller sleeves in the right nap for the texture — smooth walls and knockdown are not the same.
- Painter's tape, and the patience to pull it while the paint is still soft.
- Drop cloths, cloth rather than plastic on floors you will walk on.
- Filler and sandpaper, because the wall is never as flat as it looks in daylight.
- One spare quart of the wall colour, labelled with the room, for touch-ups.
What you are buying when you pay more
The honest answer is hide — the ability to obscure what is underneath in one pass — plus flow, scrub resistance and colour retention. Cheap paint is thinner, and a third coat of it costs more in time than the difference in price ever saved.
Builder grade
Thin, and it shows: expect a third coat where a better paint would have needed two. Rarely the cheaper choice once labour is counted.
$22–$40 per gallon
Mid range
Covers in two coats on a sound surface and scrubs without burnishing. The sensible default for most rooms.
$38–$68 per gallon
Premium
Better hide, better flow, and it genuinely covers in one coat over a similar colour. Worth it on ceilings and dark changes.
$65–$115 per gallon
The arithmetic that settles it: on a room needing two gallons, moving from builder grade to mid range costs around forty dollars. A third coat costs another gallon and most of a day. If the better paint saves you that coat even half the time, it has paid for itself.
Where the premium is unambiguous is ceilings and large colour changes — the two places where hide is doing all the work and a second visit is most annoying.
Garage floor epoxy, where the coating is the cheap part
Switch the calculator to the floor coating mode and the shape of the answer changes completely. A two-car garage takes a handful of kits; what it costs is almost entirely preparation.
Epoxy does not stick to sealed, polished or contaminated concrete. It sticks to concrete that has been mechanically ground — diamond grinding, not acid etching and not pressure washing — so that the coating has an open surface to key into. That step is roughly 60 per cent of a professional job, and skipping it is why failed garage floors peel in sheets rather than wearing gradually.
This is also the one place on this page where doing it yourself changes the answer most. The material is a modest cost; the grinder is a weekend hire. Between those two you get most of the professional result — and without the grinding you get a coating that looks perfect for two years and then comes up with the tyres.
Pouring the slab before coating it? The concrete calculator covers the yards, the rebar and the control joints — and a slab cured properly takes a coating far better.
Concrete calculatorFive ways this goes wrong
01Dividing the area by 400 and buying that
That figure is for a smooth, sealed, previously painted wall. On new drywall the first coat covers half of it, so you run out in the middle of the first day with the whole room half done.
Instead — Use the real spread rate for what you are painting over, and count the first coat separately from the ones after it.
02Skipping primer on new drywall
Bare paper and joint compound absorb differently, so the finish dries patchy — dull over the joints and sheened over the paper. Extra coats repeat the problem rather than fixing it.
Instead — One coat of primer. It costs less than the finish coat it saves and it is the only thing that evens out absorption.
03Buying trim paint by the gallon
A gallon of semi-gloss is far more than any normal amount of trim needs, and it skins over in the tin long before a second job comes along.
Instead — Quarts. More per unit, less in total, and no half-full gallon hardening in the garage.
04Mixing batches mid-wall
Two gallons of the same colour tinted at different times can differ enough to see the join, and a wall is exactly where you would see it.
Instead — Buy it all at once, and box it — pour the cans together into one bucket and mix before starting.
05Epoxy on an unground slab
Cleaning and etching does not open the surface enough for the coating to key into. It looks perfect for a season or two, then lifts in sheets, taking the top of the concrete with it.
Instead — Grind it, and do the overnight plastic-sheet moisture test first. Both are cheap; neither is optional.
How the calculator works it out
- 01Measure the wallsPerimeter of the room times the ceiling height. That is the gross wall area before anything is taken out.
- 02Subtract the openingsAbout 21 ft² per door and 15 ft² per window. Small rooms with big windows lose a surprising share.
- 03Pick the real spread rateNot the number on the tin. New drywall, texture, bare wood and masonry all cover less — and the shortfall is almost entirely in the first coat.
- 04Count the coats separatelyFirst coat at the reduced rate, later coats at the sealed rate. Averaging them is what leaves you short on day one.
- 05Add primer where the surface needs itNew drywall, bare wood, masonry and big colour changes. It costs less than the finish coat it saves.
- 06Buy trim paint in quartsA gallon of trim paint rarely gets used before it skins over. Quarts cost more per unit and less in total.
The base spread rate comes from manufacturers’ published product data. The adjustments for each substrate are our own survey and they are applied to the first coat only, because that is where the difference lives — spreading them evenly across every coat would understate the first and overstate the rest. Prices separate material from labour, which on this page is the widest gap on the site. It is all on the sources page, dated.
One rounding note: gallons are rounded up, per product, because you cannot buy two thirds of a can. The surplus is not waste — label it with the room and keep it, because touch-up paint from the same batch is the only kind that disappears.
Nothing you type is sent anywhere — the whole calculation runs in your browser.
The words on the tin
Hide and spread rate are the two that decide how much you buy. Flashing is the word for what happens when primer is skipped.
- Spread rate
- Square feet a gallon covers at the right film thickness. Published as about 400 ft², measured on a smooth, sealed, previously painted wall — the best case, not the average one.
- Hide
- A paint's ability to obscure what is underneath in one coat. It is the main thing you buy when you pay more per gallon, and the reason cheap paint often needs three coats to do two coats' work.
- Sheen
- How much light the dry film reflects, from flat through eggshell and satin to semi-gloss. Higher sheen scrubs better and shows every imperfection in the wall more clearly.
- Cutting in
- Painting the edges by brush where a roller cannot reach — corners, ceilings, trim, around openings. It is the slow part of any paint job, and it does not get faster on a bigger wall.
- Primer
- A coating designed to seal and to bond rather than to look good. It evens out absorption on new surfaces, blocks stains and tannins, and grips glossy ones. Cheaper than the extra finish coat it replaces.
- Flashing
- Patchy dull and shiny areas in a finished wall, caused by uneven absorption — typically unprimed joint compound next to bare paper. No amount of finish paint fixes it; primer prevents it.
- Holiday
- A missed spot, invisible while wet and obvious once dry and lit from the side. The reason professionals work to a wet edge and check the wall from an angle before packing up.
- Profiling
- Mechanically grinding a concrete slab to open the surface before coating it. It is the step that decides whether garage floor epoxy bonds or peels, and it is most of the cost of doing it properly.
Common questions
- How much paint do I need for a 12x12 room?
- With 8 ft ceilings that is about 384 ft² of wall before openings, or roughly 340 after a door and two windows. Over a previously painted wall, two coats takes about two gallons. On new drywall the same room takes closer to three, because the first coat covers half what the tin claims.
- How many square feet does a gallon of paint cover?
- About 400 ft² on a smooth, sealed, previously painted wall — that is the manufacturer's figure and it is a best case. On new drywall the first coat covers nearer 200, on textured walls about 300, and on masonry under 200. The published number is the ceiling, not the average.
- Do I need primer?
- On new drywall, bare wood, masonry and any large colour change, yes. Primer evens out how much the surface absorbs, blocks stains and tannins, and grips glossy surfaces. It is also cheaper than the extra coat of finish paint you would otherwise need, which makes it the rare upsell that saves money.
- How much does it cost to paint a room?
- Materials for an average bedroom run $100 to $200 including primer and sundries. A professional typically charges a few dollars a square foot of surface, which puts the same room in the high hundreds. Painting has the widest gap between material and labour of anything on this site, which is why so many people do it themselves.
- Is one coat of paint enough?
- Only over a similar colour, with a premium paint, on a sound wall — and even then the finish is uneven in raking light. Two coats is the realistic default. The good news is that the second coat goes on in roughly half the time of the first, because the cutting in is already done.
- Does textured wall need more paint?
- Yes, about a quarter more. Knockdown and orange peel add real surface area that you cannot see from across the room but a roller certainly finds. Heavier textures such as popcorn ceilings can take half as much again.
- Should I subtract windows and doors?
- Yes — roughly 21 ft² for a door and 15 for a window. It genuinely saves paint. It does not save proportional time, though: cutting in around an opening is slower than rolling the wall it replaced, which is why professional pricing does not fall as fast as the area does.
- How much does it cost to epoxy a garage floor?
- Roughly $4 to $14 per square foot installed, so a two-car garage typically lands in the low thousands. Kits are a small fraction of that — about 60% of a professional job is grinding the slab, and that is the part that decides whether the coating stays down.
- Can I epoxy a garage floor myself?
- Yes, and the material cost is modest. The difference between a DIY job that lasts and one that peels is entirely preparation: the slab has to be mechanically ground, not just cleaned or acid-etched, and it has to be dry. Hiring a grinder for a weekend is the single best money you can spend on the project.
- How do I know if my garage slab is too damp for epoxy?
- Tape a two-foot square of plastic sheet tightly to the bare slab and leave it overnight. Condensation underneath in the morning means the slab is passing moisture, and a standard epoxy will lift. That test costs nothing and saves the whole job.
- How much paint do I need for trim and doors?
- Trim works out to roughly half a square foot of paintable surface per linear foot of run, and a door takes about 40 ft² for both faces and the edges. Both are small enough that quarts make more sense than gallons — trim paint usually skins over long before a gallon is used up.
- What sheen should I use where?
- Flat on ceilings, where it hides imperfections and never gets touched. Eggshell or satin on walls, where it needs to survive cleaning. Semi-gloss on trim and doors, which take handling and need to be wipeable. The higher the sheen, the more every flaw in the surface underneath shows.
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About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
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